Unit Conversion (data.unit)

emout provides bidirectional conversion between EMSES internal (normalized) units and SI units.

Prerequisites

Unit conversion requires a conversion key in the parameter file.

plasma.inp format — first line:

!!key dx=[0.5],to_c=[10000.0]

plasma.toml format:

[meta.unit_conversion]
dx = 0.5
to_c = 10000.0

• dx: Grid spacing in meters [m]

• to_c: Speed of light in EMSES normalized units

If no key is present, data.unit is None.

Using Unit Translators

Each physical quantity has a UnitTranslator accessible via data.unit.<name>:

import emout

data = emout.Emout("output_dir")

# SI → EMSES
emses_velocity = data.unit.v.trans(1e5)     # 1e5 m/s → EMSES

# EMSES → SI
si_velocity = data.unit.v.reverse(4.107)    # 4.107 EMSES → m/s

trans(value) and reverse(value)

Method	Direction	Use case
trans(x)	SI → EMSES	Setting initial conditions, comparing with theory
reverse(x)	EMSES → SI	Interpreting simulation results

Direct SI Values from Data

The .val_si property on any grid data array returns the values in SI units:

# Potential in Volts
phisp_V = data.phisp[-1].val_si

# Current density in A/m^2
j1z_A_m2 = data.j1z[-1].val_si

# Number density in /m^3
nd1p_m3 = data.nd1p[-1].val_si

This also works for sliced data:

# SI values for a 2D slice
phi_slice = data.phisp[-1, :, 32, :].val_si

SI Units in Plots

By default, plot() uses SI units for axis labels and colorbar:

data.phisp[-1, 100, :, :].plot()              # SI units (default)
data.phisp[-1, 100, :, :].plot(use_si=False)  # EMSES units

Available Unit Translators

Name	Quantity	SI Unit
phi	Potential	V
E	Electric field	V/m
B	Magnetic flux density	T
J	Current density	A/m^2
n	Number density	/m^3
rho	Charge density	C/m^3
v	Velocity	m/s
t	Time	s
f	Frequency	Hz
length	Length	m
q	Charge	C
m	Mass	kg
W	Energy	J
w	Energy density	J/m^3
P	Power	W
T	Temperature	K
F	Force	N
a	Acceleration	m/s^2
i	Current	A
N	Flux	/m^2s
c	Speed of light	m/s
eps	Permittivity	F/m
mu	Permeability	H/m
C	Capacitance	F
L	Inductance	H
G	Conductance	S
q_m	Charge-to-mass ratio	C/kg
qe	Elementary charge	C
qe_me	Electron charge-to-mass ratio	C/kg
kB	Boltzmann constant	J/K
e0	Vacuum permittivity	F/m
m0	Vacuum permeability	N/A^2

Custom Time-Axis Units

By default, the time axis in plots uses seconds (SI). You can switch to plasma-frequency-normalized time ($\omega_{pe} t$):

from emout.emout.units import wpet_unit

# Register globally for all subsequent plots
emout.Emout.name2unit["t"] = wpet_unit

Combining Translators

UnitTranslator objects support multiplication to compose conversions:

# Time axis in output steps → SI seconds
t_translator = data.unit.t * UnitTranslator(data.inp.ifdiag * data.inp.dt, 1)